The present invention relates generally to hydraulic drive systems and more specifically relates to an auxiliary hydraulic drive system for an agricultural combine.
Generally, a combine with which the present invention concerns itself comprises a vehicle body supported on a pair of main traction wheels and a pair of rear steerable wheels. Under most conditions, the main drive wheels propel the combine. However, under some conditions, the main drive wheels may spin out and not provide the tractive force necessary. This is particularly true in the case of a combine wherein the weight of grain in the grain tank varies to a considerable extent and may vary, by more than two to one, the weight on the rear steerable wheels.
As the weight of the rear steerable wheels increases and the main drive wheels become unable to provide sufficient tractive force, it is desirable to drive the steerable rear wheels to provide the additional force necessary to propel the combine, such as described in U.S. Pat. No. 3,736,732 issued in 1973 to Jennings et al and assigned to the assignee of this invention. It would further be desirable to provide such a system with the capability of using auxiliary drive units which have a lower pressure rating than that of the main drive unit.